1. Field of the Invention
The present invention relates to a plastic optical fiber having a distributed refractive index. More particularly, the invention relates to a method of manufacturing a preform used for the production of a refractive index-distributed type plastic optical fiber. The preform thus prepared has a core and a cladding portion. The invention concerns a method of preform preparation, by virtue of which the core portion has a different refractive index between its central and peripheral zone, and in particular, the refractive-index distribution is not flattened near the central zone of the core.
2. Description of Background Information
An example of such a method is disclosed in Japanese Patent Application, published under the number Sho 61-130 904, according to which a cylindrical polymer vessel is prepared and serves as a cladding portion and then a monomer solution is put therein and polymerized and hardened so as to form a core portion. As a feature of this method, when monomer methyl methacrylate (MMA), for example, is to be used as the main monomer, a mixed solution of the monomer MMA with another monomer having a lower reactivity than the monomer MMA and a higher refractive index than a poly(methyl methacrylate) (PMMA) is used as solution for forming a core portion. In this method, the higher reactivity monomer MMA starts to polymerize from the inner surface of the cylindrical polymer vessel, whereas the lower reactivity monomer tends to copolymerize towards the center zone of the core portion, so that the refractive index thereof is graded.
Another method for manufacturing a preform for a plastic optical fiber is described in Japanese Patent Application published under the number Hei 5-173 026. According to this method, a cylindrical polymer vessel is first prepared as a cladding portion. Then, a monomer solution is polymerized and solidified inside the cylindrical vessel to form a core portion. Then, a monomer solution is polymerized and solidified inside the cylindrical vessel to form a core portion. As described in the above-mentioned Publication Sho 61-130 904, polymerization is propagated from the inner surface of the cylindrical vessel toward the center zone, in view of forming a continuously varying refractive index distribution. The feature of this method, however, is to use a second component that is a polymerizable compound having a larger molecular size than that of the first component monomer. As the polymerization starts from the inner surface of the tube, the second monomer component having a larger molecular size is pushed towards the center zone, thereby forming a higher density onwards, and thus grading the refractive index.
Also in the method disclosed in the application under the number WO 93/08 488, a monomer is polymerized from the inner surface of a cylindrical vessel consisting of a polymer, in view of forming a graded refractive index therein. For example, a cylindrical vessel is made of a PMMA tube and a core portion is formed inside the cavity of the PMA tube. The PMMA tube is prepared by putting a MMA monomer solution into a glass tube and polymerizing the monomer under heat while rotating the glass tube. To prepare a core portion, a MMA monomer solution containing a non-polymerizable compound is put into the PMMA tube and polymerized at heat under rotation. A preform having a graded index is thus obtained. The preform obtained is heat-melted and drawn, to obtain an optical fiber having a predetermined diameter.
The process disclosed in the Application WO 93/08 488 has a common point to the above-mentioned two Japanese Patent Applications, in that a polymer tube serves as a cladding portion, inside which there is formed a core portion having a continuously varying refractive index. The only difference is that a non-polymerizable compound is used in order to form a refractive index grading inside the core. When a monomer solution as a starting material for the core portion is put into the cylindrical polymer vessel, the monomer partially dissolves the inner surface of the polymer tube. Due to the gel effect, the polymerization propagates from the inner surface, where the viscosity becomes higher, to the center zone. The nearer the position to the center zone is, the higher the concentration of the non-polymerizable, high-refractive-index compound is. The refractive index is thus continuously graded. As for the polymer forming a cylindrical vessel for cladding portion, it must be formed partially or mostly from the same monomer as that for the core portion, and be soluble in the monomer solution which serves as the starting material for the core portion.
The method disclosed in the Application WO 93/08 488 is more suitable than the methods according to the other two published Applications, in that the finally obtained fiber has a smaller transmission loss. In the latter methods, as the core portion is made of a copolymer, a micro-phase separation may occur corresponding to the chain distribution of two components. This causes an increase of transmission loss. Especially, in the method described in Application Sho 61-130 904, a low-reactivity monomer is concentrated near the core center zone and remains there non-reacted due to its low reactivity. On the other hand, in the method described in WO 93/08 488, the polymer constituting the core portion is a single polymer which contains a non-polymerizable compound. Consequently, there may occur no micro-phase separation due to the chain distribution. The transmission loss may thus be minimized.
In the method disclosed in the Application WO 93/08 488, the difference An in refractive index between the center zone and peripheral zone of the core portion depends on the concentration of non-polymerizable compounds in the core-portion polymer. To increase the difference An in refractive index, the concentration of non-polymerizable compounds must be raised. However, if the concentration of the non-polymerizable compounds, which are in liquid state, is increased, the glass transition temperature of the polymer is lowered and the polymer is plasticized. Then, the distribution of refractive index in the core portion is flattened, leading to a quasi-absence of refractive index distribution and the reduction of transmission band area.
As shown in Table 1, the glass transition temperature of PMMA varies depending on the concentration of diphenylsulfide as a non-polymerizable compound.
TABLE 1 ______________________________________ Concentration of the non-polymerizable glass transition compound (wt %) temperature (.degree. C.) ______________________________________ 0 100 5 87 10 75 15 66 20 59 25 42 30 31 ______________________________________
A plastic optical fiber has a core diameter greater than that of a quartz-type optical fiber, so that the former has more propagation modes than the latter. Light propagation modes in the cladding portion (cladding mode) or near the interface core portion/cladding portion, narrow the area of propagation band and render other features less stable. These modes should therefore be reduced or removed. In order to reduce the cladding mode, the cladding layer must be made thinner. This means, if applied to the prior art, that the wall thickness of the cylindrical polymer vessel, used when preparing a preform, must be made thinner. However, this will render manufacturing of the preform more difficult.